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United States Patent |
5,646,335
|
Wenman
,   et al.
|
July 8, 1997
|
Wickless temperature controlling apparatus and method for use with pore
volume and surface area analyzers
Abstract
A wickless temperature controlling apparatus and method to analyze the
characteristics of an adsorbent is provided. The apparatus provides a heat
transfer between a cooling liquid and a sample vessel immersed in the
cooling liquid and further provides a stabilized temperature to the sample
vessel. The apparatus further comprises a thermal bridge between a
saturation vapor pressure thermometer, which is also immersed in the
cooling liquid, and the sample vessel to provide a uniform temperature
between the thermometer and the vessel. The method to analyze the
adsorbent utilizes the apparatus. In addition, the method includes a novel
method of dosing an adsorptive gas to an adsorbent.
Inventors:
|
Wenman; Richard A. (Coral Springs, FL);
Fong; Jon (Manhattan Beach, CA)
|
Assignee:
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Coulter Corporation (Miami, FL)
|
Appl. No.:
|
519054 |
Filed:
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August 24, 1995 |
Current U.S. Class: |
73/38; 73/865.5 |
Intern'l Class: |
G01N 015/08 |
Field of Search: |
73/19.05,23.27,25.01,304.11,19.01,863.11,865.5,38
422/69
220/414
215/12.1,12.2
|
References Cited
U.S. Patent Documents
8324 | Jul., 1851 | Norris | 215/12.
|
Re33567 | Apr., 1991 | Killip et al. | 73/863.
|
2376710 | Apr., 1945 | Maurer | 220/414.
|
2692497 | Oct., 1954 | Van Nordstrand | 73/19.
|
2729969 | Jan., 1956 | Innes | 73/19.
|
3059478 | Oct., 1962 | Coggeshall et al. | 73/865.
|
3211007 | Oct., 1965 | Atkins | 73/865.
|
3464273 | Sep., 1969 | Hendrix et al. | 73/865.
|
3850040 | Nov., 1974 | Orr, Jr. et al. | 73/865.
|
3934117 | Jan., 1976 | Schladitz | 219/382.
|
4304719 | Dec., 1981 | Wynne et al. | 260/314.
|
4332290 | Jun., 1982 | Skala | 165/10.
|
4489593 | Dec., 1984 | Pieters et al. | 73/38.
|
4560288 | Dec., 1985 | Nara | 374/201.
|
4693124 | Sep., 1987 | Killip et al. | 73/863.
|
4972730 | Nov., 1990 | Camp et al. | 73/865.
|
5092183 | Mar., 1992 | Leichnitz | 73/863.
|
5133219 | Jul., 1992 | Camp | 73/865.
|
5228703 | Jul., 1993 | White | 277/212.
|
5235184 | Aug., 1993 | Paulson | 250/238.
|
5237836 | Aug., 1993 | Byrne et al. | 62/385.
|
5239482 | Aug., 1993 | Akpt et al. | 364/497.
|
5360743 | Nov., 1994 | Lowell | 436/5.
|
5408864 | Apr., 1995 | Wenman | 73/38.
|
Other References
Temperature-Compensated, Differential Tensimeter for Measuring Gas
Adsorption by Low Surface Area Solids. Isao Suzuki, Rev. Sci. Instrum.
53(7), Jul. 1982, pp. 1061-1066 and Notes, p. 165.
British Standards Institute, BS 4359: Part 1: 1984, "Determination of the
Specific Surface Area of Powders-Part I Recommendations for Gas Adsorption
(BET) Methods".
Dollimore, D., et al., "The BET Method of Analysis of Gas Adsorption Data
and Its Relevance to the Calculation of Surface Areas," Surface
Technology, vol. 4, pp. 121-160 (1976).
|
Primary Examiner: Williams; Hezron E.
Assistant Examiner: Politzer; Jay L.
Attorney, Agent or Firm: Alter; Mitchell E.
Claims
We claim:
1. An apparatus used in measurement of morphological characteristics of a
solid sample contained in a vessel comprising:
a means for providing a heat transfer between a cooling liquid and a solid
sample contained in a sample vessel, said sample vessel being partially
immersed in said liquid and extending above the surface of said cooling
liquid, and wherein said means for providing a heat transfer is a heat
conductive metallic material which extends from said sample vessel into
said cooling liquid, and wherein said means for providing a heat transfer
remains below the surface of said cooling liquid without being exposed to
the atmosphere during measurement of morphological characteristics of said
sample, and provides a stabilized temperature to said sample vessel.
2. The apparatus of claim 1 which further comprises a second means for
providing a heat transfer, which is a heat conductive material that
extends from a saturation vapor pressure thermometer into said cooling
liquid, and said second means for providing a heat transfer which remains
below the surface of said cooling liquid during said measurement of
morphological characteristics of said sample, and wherein the second means
for providing a heat transfer is apart from said means for heat transfer
which extends from said sample vessel.
3. The apparatus of claim 1 wherein a first portion of said means for
providing a heat transfer is attached to a saturation vapor pressure
thermometer and a second portion of said means for providing a heat
transfer is attached to said sample vessel to provide a uniform
temperature between said thermometer and said sample vessel.
4. The apparatus of claim 3 wherein said first end of said means for
providing a heat transfer comprises a first collar to insert said sample
vessel, said first collar extending around said sample vessel, and said
second end of said means for providing a heat transfer comprises a second
collar to insert said saturation vapor pressure thermometer, said second
collar extending around said saturation vapor pressure thermometer.
5. The apparatus of claim 1 wherein said means for providing a heat
transfer is constructed of an element selected from the group consisting
of copper, silver, gold and alloys of steel and brass.
6. The apparatus of claim 5 wherein said means for providing a heat
transfer contains comprises a braided heat conductive material.
7. The apparatus of claim 5 wherein said means for providing a heat
transfer comprises a heat conductive material wound around said sample
vessel.
8. A method of determining a sorption isotherm comprising the steps of:
a) inserting a sample to be analyzed into a sample vessel and inserting
said sample vessel into a container of volatile cooling liquid, wherein a
portion of said sample vessel extends above the surface of said cooling
liquid and, said sample vessel contains an exterior metallic means that
extends from said sample vessel and remains below the surface of said
cooling liquid during analysis of said sample, said means providing a heat
transfer between said cooling liquid and said sample vessel and further
providing a stabilized temperature to said sample vessel;
b) dosing an amount of adsorbent gas to said sample; and
c) constructing a sorption isotherm resulting from the amount of said
adsorptive gas that is condensed on said sample.
9. The method of claim 8, which further comprises determining the amount of
said absorbent gas that is to be dosed to said sample from a previously
measured sorption measurement of a sample that has the same composition as
the sample being analyzed.
10. The method of claim 9, which further comprises measuring the
temperature of said cooling liquid with a saturation vapor pressure
thermometer after dosing an amount of adsorbent gas to said sample.
11. The method of claim 10, wherein said saturation vapor pressure
thermometer contains an exterior means for providing a heat transfer which
extends from said saturation vapor pressure thermometer into said cooling
liquid and remains below the surface of said cooling liquid during
analysis of said sample, said means for providing a heat transfer
providing a heat transfer between said cooling liquid and said vapor
pressure thermometer and further providing a stabilized temperature to
said thermometer.
12. The method of claim 11, wherein said external means for providing a
heat transfer on said vessel and external means for providing a heat
transfer on said saturation vapor pressure thermometer provides a thermal
bridge between said vessel and said thermometer to provide a uniform
temperature between said vessel and said thermometer.
Description
BACKGROUND OF THE INVENTION
The measurement of morphological characteristics of solids, such as
catalysts, catalyst supports, pigments, clays, minerals, pharmaceutics,
and composite materials is an important aspect of analytical chemistry and
quality control for manufacturing of numerous products.
For example, a very useful morphological characteristic of a solid is its
surface area. One of the most widely used techniques for surface area
determination is that of gas sorption. Gas sorption techniques utilize a
theoretical model wherein the surface of a solid, the adsorbent, is
characterized as being covered by a monolayer of closely packed molecules
of an adsorbed gas. After adsorption on to the adsorbent, the condensed,
relatively non-mobile gas phase is referred to as the adsorbate; whereas,
the highly mobile gaseous phase is referred to as the adsorptive. If one
can determine the amount, usually expressed in millimoles, of adsorbate
which forms the monolayer, the area which is covered by the monolayer can
be calculated from the product of the number of molecules in the monolayer
and the cross sectional area of each molecule. In 1938 Branauer, EmBett,
and Teller (J. Am. Chem. Soc., Vol. 60, 2309) described a mathematical
equation, referred to as the BET equation, for determining the amount of
adsorbate in the monolayer from the adsorption isotherm of the adsorbate.
The adsorption isotherm is a plot of the amount of the adsorbate adsorbed
on a solid adsorbent against either the relative pressure or the
equilibrium pressure of the adsorbate at a constant temperature. In order
to utilize the BET equation accurately to determine surface area, one must
at least obtain a sufficient number of data points on the adsorption
isotherm to be able to determine the point on the adsorption isotherm at
which the "monolayer capacity" occurs. The "monolayer capacity" is a
variable in the BET equation and represents the point on the adsorption
isotherm, wherein a monolayer of closely packed adsorbed molecules is
present at the surface of the adsorbent. Since the monolayer capacity
generally occurred at prior to reaching an adsorptive relative pressure of
0.35, one desires to know the adsorption isotherm at least up to this
value of relative pressure to be able to calculate the surface area from
the BET equation. The adsorptive relative pressure is one way of
expressing the equilibrium pressure of the adsorptive as a fraction of the
pressure at which bulk condensation of the adsorptive occurs under any set
of constant volume and temperature conditions. A concise review of the BET
method appears in a publication by the British Standard Institution, BS
4359: Part 1: 1984, titled, "Determination of the Specific Surface Area of
Powders-Part I Recommendations for Gas Adsorption (BET) Methods."
Adsorption isotherms can be determined by measuring the sample pressure and
determining the amount of adsorbate adsorbed either with a volumetric or a
gravimetric method. Use of this invention is applicable to both volumetric
and gravimetric determinations of isotherms, although volumetric methods
are preferred.
Three main volumetric techniques are in common use. These can be classified
as static or fully equilibrated, continuous flow or quasi-equilibrated,
and dynamic or chromatographic. Both the static and continuous flow
techniques can be described as vacuum volumetric methods. However, in some
publications, the dynamic technique has been described as continuous flow,
although it does not use vacuum technology, but instead employs a
non-adsorbing carrier gas and adsorptive mixture. For the purposes of this
invention, the term "dynamic" is applied to all of the chromatographic
types of sorption methods commonly used for rapid quality control
analysis. As related to this invention, the static and continuous flow
techniques are particularly applicable.
Volumetric methods conventionally employ a selected adsorptive at the most
convenient temperature for adsorption. For example, when one uses the
adsorptive nitrogen with an adsorbent sample to be tested, the adsorptive
is cooled to a temperature of approximately 77K. The temperature of the
adsorptive is provided by means of a liquid nitrogen bath in a dewar which
is open to the atmosphere, and therefore the adsorptive has a boiling
point equal to the environmental atmospheric pressure. By definition, the
boiling point of a liquid is the temperature of the liquid at which its
vapor pressure is equal to atmospheric pressure. When using liquid
nitrogen, the boiling point is at a temperature at which the vapor
pressure of the liquid nitrogen is equal to or slightly greater than the
atmospheric pressure of the environment.
Consequently, since both the adsorbent and the adsorptive are cooled with
the liquid nitrogen bath which is open to the atmosphere, the adsorptive
gas and the adsorbent sample have a temperature equal to the boiling point
of the liquid nitrogen bath. Because of variations in atmospheric pressure
which affect the open dewar, or impurities in the liquid nitrogen bath
which affect the saturation vapor pressure of the liquid nitrogen, the
normal boiling point temperature of the liquid nitrogen changes very
slightly from those reported at exactly one atmosphere pressure.
The reason that the adsorptive is cooled to the boiling point of liquid
nitrogen is because it is recognized by those skilled in the art that the
quantity of physically adsorbed gas at a given relative pressure (a
fraction of saturation pressure) increases with decreasing temperature.
Consequently, consistent with practical limits, the lowest most
conveniently achieved temperature is chosen to provide maximum measurement
sensitivity.
Volumetric devices typically consist of a gas storage unit and a vacuum
source unit connected in parallel to a volumetric measuring device of
known Volume V.sub.1, referred to as the doser unit or the manifold unit.
The doser unit can be connected alternately to either the vacuum unit or
the gas storage unit by a series of valves. The doser unit in turn is
connected in series through another valve to a sample unit, a chamber of
known Volume V.sub.2, which holds the solid sample to be tested. By
manipulating the appropriate valves, the doser and sample units are
evacuated and the evacuated doser is sealed off from the evacuated sample
chamber. Nitrogen is permitted to slowly enter and fill the doser unit
from the gas storage unit to a targeted pressure, at which time the valves
are closed to seal off the doser, and the nitrogen pressure therein is
measured. When a constant pressure P.sub.1 in the doser is achieved, the
valve separating the sample chamber and doser is opened allowing the
adsorptive, typically N.sub.2, in the doser to expand into the sample
chamber.
The sample chamber and doser together define a third Volume V.sub.3 (i.e.
V.sub.1 +V.sub.2). When the pressure in V.sub.3 is constant, indicative of
adsorption equilibrium, it is measured. This equilibrium pressure is used
to calculate the total number of moles of N.sub.2 that remains in the gas
phase. The number of moles of N.sub.2 adsorbed on the solid is equal to
the number of moles of N.sub.2 initially present in Volume V.sub.1 of the
doser, plus the number of moles of N.sub.2 in the sample chamber defining
Volume V.sub.2 (the number of moles in Volume V.sub.1 for the initial run
is O, but increases with each successive run), less the number of moles of
gaseous N.sub.2 in Volume V.sub.3, after equilibrium. The combined data of
the amount of N.sub.2 adsorbed at a particular equilibrium pressure
constitutes a single point on the adsorption isotherm. The above procedure
is repeated to obtain additional points on the adsorption isotherm. Each
successive dose increases the pressure in the sample chamber until, at
approximately atmospheric pressure, the sample becomes completely
saturated with condensed N.sub.2. At this saturation point, the majority
of the N.sub.2 condensation occurs on the sample contained in the sample
holder. Conventional practice is to generate about 3 to 10 data points on
the adsorption isotherm for surface area determinations. A detailed
summary of this method is provided in the review paper, "The BET Method of
Analysis of Gas Adsorption Data and Its Relevance to the Calculation of
Surface Areas" by Dollimore, D., Sponner, P., and Turner, A. Surface
Technology, Vol. 4, p. 121-160 (1976).
However, with manual dosing methods, exact target pressures are rarely
achieved and several additional unwanted data points can be obtained.
While the adsorptive gas is being dosed to the manifold in order to reach
an estimated required pressure, an increase above the pressure desired can
be obtained. If this higher than required pressure is dosed to the sample,
significant loss of operational range can result if too much gas is
adsorbed. It is normal practice to open the nitrogen gas adsorptive valve
very carefully. This valve typically is capable of supplying nitrogen to
higher than atmospheric pressure. If an increase above the pressure
desired does occur, then it must be removed by judicious opening of the
vacuum valve, before adsorption. Additional pressure stabilizing time is
always required and typically is equal to one or two minutes. This time is
in addition to the sorption equilibration time.
A capillary method having a continuous flow of the adsorptive is disclosed
in U.S. Pat. No. 2,729,969 to Innes. Innes teaches a method for the
measurement of surface areas which comprises introducing nitrogen gas at a
constant flow rate into an evacuated system containing a weighed sample
amount which is cooled to -195 C., measuring the time required for the
vacuum in the evacuated system to decrease from 29.6" to 23.7" of mercury
and calculating from the time required the surface area of the material.
Innes further teaches that due to impurities present in the liquid
nitrogen cooling bath, the bath temperature is somewhat higher than the
boiling point of pure nitrogen. As a result, the saturation vapor pressure
is slightly above one atmosphere. It has been reported elsewhere that
dissolved impurities usually increase the bath temperature sufficiently to
cause the vapor pressure of pure liquid nitrogen in the sample cell to
increase by 10 to 20 mm Hg above ambient pressure. However, the Innes
method suffers from environmental induced flow rate fluctuations and
imprecise equilibrium pressure conditions, which affect the accuracy of
measurement.
U.S. Pat. No. 4,489,593 to Pieters, et al. discloses using an adsorptive at
a temperature at which the adsorptive condenses at approximately one
atmosphere pressure and introducing the adsorptive into the sample holder
containing the substance to be analyzed at a constant flow rate. By
controlling the mass flow rate to be not greater than the equilibration
rate of adsorption, the pressure established, at any given time during the
introduction of the adsorptive, will be equilibrium pressure. This is
significant because the adsorption isotherm is a plot of the amount of
adsorptive adsorbed at a given equilibrium pressure. Consequently the
determination of the adsorption isotherm is simplified.
U.S. Pat. No. 4,972,730 is directed to the determination of the saturation
vapor pressure after dosing and measurement of the adsorption isotherm has
been completed. The sample is deliberately overdosed to cause bulk
condensation of adsorbate in the sample tube. This condensation occurs
exterior to the sample pores because the pores have already been filled
with liquid gas during the measurement of the adsorption isotherm. At the
point when the sample pores are full, the addition of further gas must
condense as bulk liquid. Measurement of the sample pressure after this
overdose gives an accurate determination of the saturation vapor pressure
at the sample temperature (at this moment in time only). The saturation
vapor value recorded with this sample tube vapor pressure thermometer is
used for the calculation of relative pressure values during the subsequent
measurement of the desorption isotherm. However, the sample tube vapor
pressure thermometer suffers from thermal currents in the liquid nitrogen
bath.
In each of the methods described above, it is important to have a non
fluctuating temperature because uncontrolled changes in temperature create
uncontrolled changes in saturation vapor pressure. This can lead to errors
in determining the amount of gas adsorbed by the sample.
Several prior art devices have been developed to maintain gases at a
constant temperature within a vessel. In one device, the cryogenic liquid
is contained within a dewar flask, and the entire flask is raised at the
same rate that evaporation occurs, thereby keeping the level of the liquid
at the same height with respect to the vessel. The major disadvantage to
this device is that, as the dewar flask is raised, it surrounds more and
more of the previously exposed portion of the vessel and traps the cold
gas molecules as they evaporate from the cryogenic liquid, thereby
shifting the gradient and creating uncontrolled temperature and pressure
changes within the vessel.
Another device, described in U.S. Pat. No. 3,850,040, transfers fresh
liquid to the dewar flask at the same rate that evaporation occurs, so
that the height of the liquid remains at a constant level. In this device
the temperature gradient does not shift. The major disadvantage to this
device is that, in humid climate, ice can accumulate within the valves and
seals on the apparatus used to pump the liquid from the reservoir to the
dewar flask and can cause the device to fail.
An apparatus for use with a scientific instrument such as a pore volume and
surface area analyzer is disclosed in Killip et al. U.S. Pat. No.
4,693,124. Killip et al. teach maintaining gases within the sample vessel
at constant temperature, by providing an apparatus which maintains a
liquid at a fixed height surrounding the gas-containing vessel. Killip et
al. teach maintaining a constant temperature within a vessel immersed in a
liquid by surrounding a portion of the vessel, extending above the surface
of the liquid, with a wick. The wick conducts liquid up to a predetermined
point on the vessel and maintains the liquid at that point, regardless of
changes in the level of the liquid due to evaporation.
Desorption isotherms are important because various mathematical equations
have been developed to enable one skilled in the art to determine certain
morphological characteristics of solids. More specifically, adsorption, as
well as desorption isotherms enable one to calculate the pore size
distribution of a solid sample from the data embodied therein. A
desorption isotherm is a plot of the amount of a preadsorbed gaseous
material, the desorbate, desorbed from a solid against the equilibrium
pressure or relative pressure of the desorbate at a constant temperature.
After desorption from a solid sample, the desorbent, the gas is referred
to as the desorptive. The desorption isotherm differs from the adsorption
isotherm in that it is constructed starting with a solid, saturated with
the desorbate, and gradually reducing the pressure over the solid to near
absolute vacuum. In contrast, the adsorption isotherm starts with an
evacuated solid sample and increases the pressure of a gaseous adsorptive
in contact therewith sample saturation is reached. The adsorption and
desorption isotherms are collectively known as the isotherm.
Gas-solid interaction can cause at least a portion of the desorption path
of the sorption isotherm to lie higher on the isotherm plot than the
adsorption path. The failure of the desorption path to duplicate the
adsorption path of the isotherm is referred to as hysteresis. The two most
common forms of hysteresis are closed loop and open loop. In the closed
loop hysteresis behavior, the desorption path of the isotherm eventually
rejoins the adsorption path at some low relative pressure. Closed loop
hysteresis normally is associated with porosity in the sample being
tested.
For example, at the start of the desorption isotherm, the pores of the
sample are saturated and filled with the desorbate. As desorption occurs,
capillary action delays desorption of the desorbate present within the
pores, such that a lower pressure is required to evacuate the pores
relative to the pressure which initiated the filling of the pores during
adsorption. This delay is expressed as closed loop hysteresis behavior of
the sorption isotherm. Open loop hysteresis is characterized by the
failure of the desorption path of the isotherm to rejoin with the
adsorption path. Open loop hysteresis usually is associated with some
measurable amount of irreversible adsorption, which typically occurs when
the gas reacts with the solid sample upon adsorption, conventionally
referred to as chemisorption. Consequently on desorption, less material
will desorb than was initially adsorbed, giving rise to an open loop in
the sorption isotherm.
By intentionally inducing chemisorption, much can be learned about the
surface of the solid sample. For example, chemisorption can be employed to
determine the percent dispersion and surface area of microscopic particles
of a catalyst deposited on a support by employing a gaseous adsorbate
which will undergo chemisorption with the catalyst particles but not the
support.
Other information in the substantially complete sorption isotherm permits
the determination of total pore volume, average pore size, and pore shape,
for example, slits versus cylindrical pores.
The above discussion highlights only a few of the incentives for obtaining
substantially complete pictures of the entire sorption isotherm rather
than narrow segments thereof, and any method or device capable of
producing substantially complete sorption isotherms quickly and accurately
possesses substantial advantages.
In view of the above, it is evident that there has been a continuing search
for a simple device that maintains an evaporating liquid at a constant
temperature in order to keep the gases within a vessel immersed in the
liquid at a constant temperature. The present invention was developed in
response to this search.
SUMMARY OF INVENTION
The present invention provides a temperature controlling apparatus and
method for use with surface area and pore volume analyzers. The apparatus
comprises a container of volatile cooling liquid, such as a dewar flask
filled with liquid nitrogen, a sample vessel which contains a sample to be
analyzed, and a temperature controlling means for providing a heat
transfer between the cooling liquid and the vessel and further providing a
stabilized temperature to the vessel. The sample vessel is immersed in the
cooling liquid, but extends above the surface of the cooling liquid. The
means remains immersed in the volatile cooling liquid without being
exposed to the atmosphere regardless of changes in the level of the
cooling liquid.
Once immersed into the cooling liquid, the temperature controlling means
remains stationary and is wickless. More specifically, once the sample
vessel is attached to the means, it does not move in the cooling liquid.
Moreover, the means does not conduct the cooling liquid positioned in the
container from a location at or below the surface of the liquid in the
container to a point along the side of the vessel which is above the
surface of the liquid.
The apparatus further comprises a thermal bridge between a saturation vapor
pressure thermometer, which is also immersed in the cooling liquid, and
the sample vessel to provide a uniform temperature between the thermometer
and the vessel.
The method of this invention provides a method of determining a sorption
isotherm comprising the steps of inserting a sample to be analyzed into a
sample vessel which contains an exterior means that remains below the
surface of the cooling liquid during analysis of the sample; providing a
heat transfer between a cooling liquid and the sample vessel and further
providing a stabilized temperature to the vessel through the exterior
means; immersing the vessel in a container of volatile cooling liquid with
a portion of the vessel extending above the surface of the cooling liquid;
dosing an amount of absorbent gas to the sample; and constructing a
sorption isotherm from the amount of said adsorptive gas that is condensed
on the sample.
In a preferred embodiment, the amount of gas used to dose the sample is
determined from a previously measured sorption measurement of a sample
that is the same as the sample being analyzed.
The apparatus and method are useful for providing accurate and reproducible
pore size determinations.
BRIEF DESCRIPTION OF TEE DRAWINGS
FIG. 1 is a vertical cross section view of the temperature controlling
apparatus embodying the present invention.
FIG. 2 is a vertical cross section view of a second embodiment of the
present invention.
FIG. 3 is a vertical cross section view of a third embodiment of the
present invention.
FIG. 4 is an isotherm plot which is constructed with sample pressure rather
than relative pressure.
DETAILED DESCRIPTION OF TEE INVENTION
The present invention is related to an apparatus and method to improve gas
sorption measurements. Preferably, the apparatus and method provide
improved pore size distribution measurements. The apparatus and method can
be used for instruments that employ static and continuous flow techniques,
and other vacuum volumetric methods. The apparatus and method of the
present invention are applicable in an adsorption mode, in a desorption
mode, or a combination of the two, wherein the adsorption mode is followed
by the desorption mode.
The adsorption mode is conducted using a substance existing initially as a
gas vapor, referred to therein as the adsorptive, and a solid sample,
referred to herein as the adsorbent or sample. During the course of the
adsorption mode, the adsorptive is adsorbed by the adsorbent. Thereafter,
the adsorptive that is adsorbed is referred to as the adsorbate. The
identity of the adsorptive will vary depending on whether the nature of
the adsorption is intended and controlled to be physical, or physical and
chemical. It is known that the adsorption phenomena can be the result of a
physical or a physical and chemical process depending on the system
involved and the temperature employed. Physical adsorption, frequently
referred to as van de Waals' adsorption, is the result of a relatively
weak interaction between solid and gas. One of the characteristics of this
type of adsorption is that all the gas adsorbed by the solid can be
removed therefrom by evacuation at about the same temperature at which it
was adsorbed. For physical adsorption, the adsorptive is selected to be
chemically inert with respect to the adsorbent.
Representative examples of adsorptives conventionally employed for physical
adsorption include nitrogen, argon, krypton, oxygen, xenon, neon, helium,
carbon dioxide and hydrocarbons, such as methane, butane, hexane and
benzene. The adsorptive gas can comprise a component gas and an inert
carrier gas. Preferably the adsorptive gas comprises at least 80 percent
of a single component gas and less than 20 percent of a carrier gas. Most
preferably, the adsorptive gas comprises a single gas without a carrier
gas.
Since the quantity of physically adsorbed gas or vapor at a given pressure
increases with decreasing temperature, the adsorptive typically is
selected so that it will liquify at very low temperatures. These low
temperatures correspond to the boiling points at atmospheric pressure of
conventionally employed adsorbates which are noted in the following table.
TABLE 1
______________________________________
Standard Boiling
Point at 1 atmosphere (K)
______________________________________
Nitrogen 77.35
Argon 87.45
Krypton 120.45
Oxygen 90.18
Xenon 166.05
Neon 27.10
Helium 4.2
Air (21% 0.sub.2)
78.8
Methane 111.66
Butane 272.5
Hexane 341.7
Benzene 353.1
Carbon dioxide
194.5
______________________________________
Before determining the amount of adsorptive adsorbed by a sample, the
sample is placed in a sample chamber and cleansed of impurities by
removing adsorbed atmospheric gases, such as nitrogen, oxygen, and water
vapor. This process is referred to as outgassing. The process is achieved
by conventional methods known to those skilled in the art. Examples of
such methods are described in Orr, C. and Dallavalle, J., "Fine Particle
Measurement" Macmillan Co., p. 164-204 (1960). One such disclosed example
includes heating the sample in a vacuum at temperatures of about
110.degree. to about 600.degree. C. for a period of from about 4 to about
12 hours. The sample weight and density can also be determined in
accordance with conventional methods before the sample is contacted with
the adsorptive.
Thereafter and in accordance with procedures known to those skilled in the
art, the overall volume of the sample chamber is determined. In addition,
the volume of the sample in the chamber is determined by procedures known
to those skilled in the art and designated as the "dead space"; whereas,
the remaining volume of the sample chamber and the piping between it and
the manifold doser is designated as the "free space." Typically, helium
gas is chosen to be used to determine the dead space, because it is
substantially inert and also because there is no appreciable adsorption of
helium by the sample materials at the cryogenic temperature of the sample.
Using methods known to those skilled in the art, the free space is
determined by the conventional method of precharging the doser unit to a
predetermined pressure, and then permitting the helium in the doser unit
to expand into the sample chamber. The reduction in pressure in the doser
unit can be converted by those skilled in the art by use of standard
computation techniques to a value which represents a corresponding number
of cubic centimeters of helium gas, taken at the reference temperature and
pressure.
After the free space of the sample chamber has been determined, sample
analysis is commenced. In traditional static and continuous flow
techniques, the adsorptive nitrogen gas and adsorbent are maintained at
approximately the boiling point temperature of the adsorptive and at a
saturation vapor pressure of one atmosphere. As appreciated by those
skilled in the art, the temperature of the adsorptive in the conventional
isotherm measurement can change slightly due to environmental conditions
in the liquid nitrogen bath. Table 2 demonstrates the gradual increase in
temperature of the liquid nitrogen bath that typically occurs due to
oxygen, carbon dioxide and water vapor contaminants which are from the
environment.
TABLE 2
______________________________________
Time-Temperature of a Liquid Nitrogen Bath
______________________________________
0 min. 77.103K 90 min. 77.128K
5 77.106 95 77.143
10 77.099 100 77.155
15 77.079 105 77.130
20 77.090 110 77.121
25 77.086 115 77.130
30 77.095 120 77.129
35 77.104 125 77.137
40 77.077 130 77.128
45 77.083 135 77.144
50 77.101 140 77.154
55 77.071 145 77.162
60 77.104 150 77.171
65 77.107 155 77.183
70 77.112 160 77.200
75 77.114 165 77.208
80 77.122 170 77.225
85 77.124 175 77.218
180 77.227
______________________________________
The fluctuations in the temperature are attributed to several factors
including impurities which occur because the dewar is open to the
atmosphere. The fluctuations in temperature causes thermal currents within
the liquid nitrogen bath. As noted, the increase in temperature of the
liquid nitrogen bath has a long term affect of increasing approximately
0.12K over a three hour period. During the short term intervals, there are
temperature variations, which although are small, they can significantly
affect surface area determinations and especially pore size
determinations.
The temperatures noted above were obtained in an open dewar containing
liquid nitrogen. The liquid nitrogen was not replenished during the
measurement. The boiling point of the liquid nitrogen was determined to be
at 769.3 mm Hg pressure. In addition, the boiling point of fresh nitrogen
was observed to be approximately 77.10K. The difference between the
observed boiling point temperature and the standard textbook value
reported, might be attributed to the accuracy of the temperature probe and
environmental atmospheric conditions.
As appreciated by one skilled in the art, the saturation vapor pressure is
determined by prior art methods so that sorption measurements can be made
at relative pressures from 0 to 1. To determine the saturation vapor
pressure, the prior art supplies a predetermined high pressure of the
adsorptive into a vapor pressure thermometer. Thereafter, the vapor
pressure thermometer is placed into a cooling medium, which causes a small
amount of the gaseous adsorptive to condense; thereby establishing the
saturation vapor pressure of the adsorptive at a temperature which is also
the temperature at which subsequent analysis of the sample chamber occurs.
As taught by U.S. Pat. No. 3,850,040 to Orr, Jr. et al., when using
nitrogen as the adsorptive, the saturation vapor pressure is determined by
precharging the vapor pressure thermometer with nitrogen to a
predetermined high pressure, such as 850 mm Hg, and thereafter placing the
vapor pressure thermometer into an open dewar filled with liquid nitrogen.
The lowering of the temperature of the vapor pressure thermometer causes a
small amount of the gaseous nitrogen in the vapor pressure thermometer to
condense, thereby establishing the saturation vapor pressure of nitrogen
at a temperature which also is used for subsequent analysis of the sample
chamber. It will also be understood that this saturation vapor pressure
for nitrogen can be used to calculate the saturation vapor pressure for
another gas in the event that such other gas is used in place of nitrogen
for the gas sorption analysis.
By using similar principles, the saturation temperature can be determined.
To determine the saturation temperature, a vapor pressure thermometer is
precharged with the adsorptive to a predetermined high pressure.
Thereafter, the vapor pressure thermometer is cooled by the cooling
medium. When the adsorptive is at equilibrium between the vapor and liquid
phases, the temperature is measured, which is also the saturation
temperature used in subsequent analysis of the sample chamber.
The identity of the adsorbent or sample can be any solid sought to be
analyzed for its morphological characteristics, such as surface area. The
apparatus and method described herein are applicable to a sample having a
surface area of typically from about 0.001 to about 2000 m.sup.2 /g,
preferably from about 0.05 to about 1500 m.sup.2 /g, and most preferably
from about 0.5 to about 1000 m.sup.2 /g; and pore size radii of typically
from about 0.35 to about 300 nanometers, preferably from about 0.5 to
about 100 nanometers, and most preferably from about 0.5 to about 50
nanometers.
The apparatus of the present invention is used to provide a stabilized
temperature to the sample vessel. A stabilized temperature means that the
sample temperature does not fluctuate during short term intervals due to
thermal currents. As previously noted in Table 2, the cooling liquid
temperature has short term positive and negative fluctuations of
temperature from the previous reading throughout a typical period of
analysis. However, the apparatus provides a stabilized temperature to the
sample vessel which is unaffected by the heat transfer from the: 1)
adsorption and desorption process, 2) atmosphere to the sample by the
vessel, or 3) gas dose. Any one of these three items can cause short term
differences in temperature to the sample vessel. Notwithstanding, it is
appreciated by those skilled in the art, that the sample temperature will
be affected by the long term changes of the cooling liquid temperature.
The apparatus of the present invention that is depicted in the drawings.
Referring now in more detail to the drawings, in which like reference
numerals refer to like parts throughout the several views, FIG. 1 shows a
temperature controlling apparatus 10 embodying the present invention. The
apparatus includes a container 12, preferably an insulated container such
as a dewar flask, having an external wall 14 surrounding a vacuum chamber
16. It will be understood that any container capable of withstanding
extremely cold temperature of the particular liquid to be placed therein,
can be substituted for the dewar flask, depending on the nature of the
liquid to be held in the container.
The container 12 is partially filled with a liquid 20, preferably an
evaporating cooling liquid such as liquid nitrogen. Liquid nitrogen is
preferred since it will provide a very cold temperature medium to surround
a vessel 30 at very low temperatures. The uppermost level of the liquid
defines a horizontal surface 22 from which evaporating liquid or vapor
escapes.
A test sample 25 is place into a vessel 30 for analysis. The vessel
comprises a size and shape capable of being inserted within the liquid
filled container 12. The sample containing vessel 30 is then immersed in
the liquid 20 in the container 12. The preferred vessel 30 of the present
invention includes a flat bottom flask 32 at the lowermost end of the
vessel having an opening 33 connected to an elongated tube 34 extending
upwardly from the flask. The preferred vessel 30 thus can be a flat
bottomed vessel of the type commonly used in scientific laboratories. For
some applications, such as dynamic or chromatographic flow sample
analysis, the sample vessel may be a U-shaped tube. The entire vessel is
made of a substance capable of withstanding the temperature of the liquid
into which it is immersed. Preferably, the vessel is made of a substance
capable of withstanding extremely hot or cold temperature, such as
PYREX.RTM. brand glassware.
The flat bottom flask 32 of the sample containing vessel 30 is normally
immersed below the surface 22 of the liquid 20 as shown in FIG. 1. The
elongated tube 34 can also be immersed partially below the surface 22 of
the liquid and is removably attached at it uppermost end 36 to an
analytical scientific instrument (not shown). The attached analytical
instrument can be, for example, a pore volume and surface area analyzer
such as a Coulter Corporation SA 3100 analyzer, manufactured by Coulter
Corporation, Miami, Fla. The pore volume and surface area of the sample 25
contained within the sample flask 32 of the vessel 30 is determined
according to methods known to those skilled in the art.
In accordance with conventional volumetric gas sorption analytical
procedures, the volume of the sample holder is selected to be from about 1
to about 20, preferably 1 to 10, and more preferably 1 to 5 times the
volume of the sample. The size of the sample holder contributes to an
accurate determination of the free space used for both adsorption and
desorption; and to minimize error which can be introduced by the gas
volume value at the liquid bath-air interface by fluctuations in the
liquid nitrogen level. If the sample holder is too large, the accuracy of
measurement will be adversely affected.
AS shown in FIG. 1, the elongated tube 34 of the vessel 30 is surrounded by
a sleeve or collar 50. The collar 50 extends from bottom of the elongated
tube 34 to a predetermined point beneath the surface 22 of the liquid 20.
Examples of a possible collar would include a solid metal collar, wire
wound around the flat bottom flask 32, and braided wire around the flat
bottom flask. In a preferred embodiment, not shown, the collar 50 covers
the flat bottom flask 32. More specifically, the collar 50 covers the
entire lower portion of the flat bottom flask 32 and a portion of the
elongated tube 34, providing it remains below the surface 22 of the
cooling liquid 20.
The top of the collar 50 should be always remain lower than the surface 22
of the liquid 20. Therefore, the length of the collar 50 should take into
account the amount of evaporation which will occur during the analytical
testing of the sample in the vessel. The collar 50 is comprised of a heat
conductive material which will provide an efficient heat transfer between
the liquid 20 and the test sample 25. Examples of such material would
include metals such as copper, silver, gold or alloys metals such as steel
and brass.
The collar 50 and the flat bottom flask 32 of the sample containing vessel
30 can be constructed of the same material, providing that the elongated
tube 34 that is above the liquid level 22 is of a non heat conductive
material. More specifically, the collar 50, and flask 32 can be a single
fabricated apparatus providing that it is connected to a elongated tube 34
which is a thermal insulator so that temperature from the atmosphere above
the liquid level 22 is not transferred into the liquid 20.
The collar 50 is impervious to the liquid 20 and should fit around the
flask 32 to enable an efficient heat transfer from the adsorptive gas and
the adsorbent sample to the liquid nitrogen 20. Since liquid nitrogen acts
as an insulating material, it is preferred to have a minimum amount of
liquid nitrogen between the collar 50 and the flask 32. The collar 50 can
be of varying thickness. The function of the collar 50 is to provide an
efficient heat transfer between the liquid 20 to the test sample 25. In
addition, the collar 50 provides a heat reservoir to eliminate the
temperature fluctuations caused by the thermal currents in the liquid 20.
Preferably, the collar has a thickness of approximately 0.5 millimeters.
As appreciated by those skilled in the art, the thickness can vary
dependent upon the mass of the collar.
The adsorption process is exothermic, and this additional heat increases
the convection currents that exists in the liquid nitrogen bath used to
cool the sample. A 0.25K difference causes a significant effect upon the
saturation vapor pressure and the apparent pore size. The saturation vapor
pressure of nitrogen changes by approximately 22 mm Hg for each 0.25K
change in temperature from 77.4K. For example, the apparent Kelvin radius
calculated at a relative pressure of 0.995 changes from 187 nanometers to
27.5 nanometers if the temperature change increase by 0.25K.
An isotherm plot which is constructed with sample pressure rather than
relative pressure illustrates the effect of small temperature errors on
the sample pressure. These differences can create substantial errors in
the pore size determinations. This is graphically shown in FIG. 4 which
depicts three isotherms which are developed with slightly different sample
temperatures. Consequently, the adsorbent temperature must be accurately
known for determining surface area and most importantly for pore size
determinations.
In a second embodiment of the present invention, FIG. 2 shows a temperature
controlling apparatus 100 embodying the present invention. FIG. 2 is a
vertical cross-sectional view similar to that shown in FIG. 1 for the
first embodiment. In the second embodiment, the container 12, liquid 20,
sample 25, vessel 30 and collar 50 are similar to that for the first
embodiment shown in FIG. 1, however a saturation vapor pressure
thermometer tube 70 is added to provide an accurate reading of the
temperature during the analysis.
The saturation vapor pressure thermometer tube 70 comprises a tube of a
size and shape capable of being inserted within the liquid filled
container 12. The preferred saturation vapor pressure tube 70 of the
present invention includes flat bottom cylinder 72 at the lowermost end of
the tube having an opening 73 connected to an elongated tube 74 extending
upwardly from the cylinder. However, the preferred saturation vapor
pressure thermometer tube 70 can have a rounded or flat bottomed and is of
the type commonly used in scientific laboratories. The entire tube is made
of a substance capable of withstanding the temperature of the liquid into
which it is immersed. Preferably, the tube is made of a substance capable
of withstanding extremely hot or cold temperature, such as PYREX.RTM.
brand glassware.
The tube 70 is then immersed in the liquid 20 in the container 12. As shown
in FIG. 2, the elongated tube 74 is also immersed partially below the
surface 22 of the liquid 20 and is removably attached at it uppermost end
76 to an analytical scientific instrument (not shown). As further shown in
FIG. 2, the cylinder 72 of the vessel 70 is surrounded by a sleeve or
collar 90. The collar 90 extends from bottom of the cylinder 72 to a
predetermined point beneath the surface 22 of the liquid 20. In a more
preferred embodiment, not shown, the collar 90, covers the bottom of the
cylinder 72. More specifically, the collar 90 would cover the entire lower
portion of the flat bottom cylinder 72 and a portion of the elongated tube
74, providing it remains below the surface 22 of the cooling liquid 20.
The top of the collar 90 should be always remain lower than the surface 22
of the liquid 20. Therefore, the length of the collar 90 should take into
account the amount of evaporation which will occur during the analytical
testing of the sample in the vessel. The collar 90 is comprised of a heat
conductive material which will provide an efficient heat transfer between
the liquid 20 and the adsorptive gas. Examples of such material would
include metals such as copper, silver, gold or alloys metals such as steel
and brass.
The collar 90 and the cylinder 72 of the saturation vapor pressure
thermometer 70 can be constructed of the same material, providing that the
elongated tube 74 that is above the liquid level 22 is of a non heat
conductive material. More specifically, the collar 90, and cylinder 72 can
be a single fabricated apparatus providing that it is connected to a
elongated tube 74 which is a thermal insulator so that temperature from
the atmosphere above the liquid level 22 is not transferred into the
liquid 20.
The collar 90 is impervious to the liquid 20 and should fit around the
cylinder 72 to have an efficient heat transfer between the adsorptive gas
and the liquid nitrogen 20. Since liquid nitrogen acts as an insulating
material, it is preferred to have a minimum amount of liquid nitrogen
between the collar 90 and the cylinder 72. The collar 90 can be of varying
thickness. The function of the collar 90 is to provide an efficient heat
transfer from the liquid 20 to the adsorptive gas in the tube 70 and to
provide a heat reservoir to eliminate the temperature fluctuations of the
adsorptive gas in the tube 70 caused by the thermal currents in the liquid
20. Preferably, the collar has a thickness of approximately 0.5
millimeters. As appreciated by those skilled in the art, the thickness can
vary dependent upon the mass of the collar.
In a third embodiment of the present invention, FIG. 3 shows a temperature
controlling apparatus 110 embodying the present invention. FIG. 3 is a
vertical cross-sectional view similar to that shown in FIG. 2 for the
second embodiment. In the third embodiment, the sample vessel 30 and the
saturation vapor pressure thermometer 70 are similar to that shown in FIG.
2 for the second embodiment. In the third embodiment, the sample vessel 30
and saturation vapor pressure thermometer 70 are thermally connected to
each other to provide a consistent temperature for the vessel 30 and the
thermometer 70. More specifically, the sample vessel 30 and the saturation
vapor pressure thermometer 70 have a heat bridge 95 that is formed of a
material which surrounds the vessel 30 and the thermometer 70.
The top of the bridge 95 should be always remain lower than the surface 22
of the liquid 20. Therefore, the height of the bridge 95 should take into
account the amount of evaporation which will occur during the analytical
testing of the sample in the vessel. The bridge 95 is comprised of a heat
conductive material which will provide an efficient heat transfer among
the liquid 20, the vessel 30 and the thermometer 70. The bridge 95 also
provides a further means to stabilize the temperature fluctuations which
occur in the vessel and thermometer. In addition, the bridge provides
still a further means by which the vessel and thermometer have similar
reportable temperatures. Examples of material that can be used to
construct the bridge 95 would include metals such as copper, silver, gold
or alloys metals such as steel and brass.
The bridge is constructed so that the flat bottom flask 32 of the sample
containing vessel 30, and cylinder 72 of the saturation vapor pressure
thermometer can be inserted into the bridge in preparation for the sample
analysis and withdrawn from the bridge 95 after analysis has been
completed. Examples of a possible bridge would include a thermal
conductive material which sandwiches the sample vessel 30 and cylinder 72
in the middle. Alternatively, the bridge 95, the flat bottom flask 32 of
the sample containing vessel 30, and cylinder 72 of the saturation vapor
pressure thermometer can be constructed of the same material, providing
that the bridge remains below the surface of the liquid level 22. More
specifically, the bridge 95, the flat bottom flask 32 of the sample
containing vessel 30, and cylinder 72 of the saturation vapor pressure
thermometer can be a single fabricated apparatus. In such case, the
elongated tube 34 and the elongated tube 74 should be a thermal insulator
so that temperature from the atmosphere above the liquid level 22 is not
transferred into the liquid 20.
The bridge 95 can be of varying thickness. The function of the bridge 95 is
to provide a heat reservoir to eliminate the temperature fluctuations of
the test sample 25 and the saturation vapor pressure thermometer 70. In
addition, the bridge provides a further means by which the vessel and
thermometer have similar reportable temperatures. Preferably, the collar
has a thickness of approximately 0.5 millimeters. As appreciated by those
skilled in the art, the thickness can vary dependent upon the mass of the
collar.
Sorption analysis is performed at one or more relative pressure points from
0 to 1. The sorption analysis can be performed at fixed pressure points
equivalent to the required relative pressures, by the incremental increase
of pressure from the pressure source. Although these fixed pressure points
can be calculated from the predetermined relative pressure points,
relative to the saturation pressure, this is not required by this
invention. More specifically, one can choose relative pressures of 0.04,
0.08, 0.12, 0.16 and 0.20; however, broader variation of relative
pressures can be used for determination of surface area and especially
pore size. That is, if the majority of relative pressure points are
between 0.05 and 0.35, one can employ additional relative pressure points
which do not correspond to the addition of the adsorptive in successive
equal increments.
As noted previously, the temperature of the adsorptive can change slightly
due to environmental conditions of the liquid nitrogen bath. More
specifically, the temperature of the liquid nitrogen, held in a dewar,
always is found to be somewhat different than the normal boiling point,
because of dissolved impurities from the atmosphere and because of ambient
pressure fluctuations. Dissolved impurities usually increase the bath
temperature sufficiently to cause the vapor pressure of pure liquid
nitrogen within the sample cell to increase above ambient pressure. The
prior art has compensated for this by adding about 15 mm Hg to ambient
pressure.
However, according to the present method, the saturation vapor pressure is
measured and the correction factor is not used became the saturation vapor
pressure thermometer measures the temperature at each point to provide an
accurate measurement of the saturation vapor pressure.
Dosing schemes known to those skilled in the art can be used with the
present invention. Alternatively, three slightly different dosing methods
can be used in this invention. The dosing alternative method employs a
philosophy that if the data point is not at the relative pressure point
desired, it should be recorded anyway, and the next dose of adsorptive gas
should be accurate to obtain the desired relative pressure point. Data
required at precise relative pressure points are obtained by interpolating
the measured data. Both sets of data (measured and interpolated) are
always available to the instrument user. Consequently, all adsorbate doses
are equilibrated and recorded. The alternative dosing method does not use
any iterative procedure within each data point.
The three alternative dosing methods are: (1) Unknown Mode, (2) Learn Mode
and (3) Copy Mode. All of these modes use a target table of relative
pressures which are programmed into the scientific instrument. The
instrument attempts to record data points at these table values, however a
mathematical interpolation is used to report very precise relative
pressure values. Additional data points greater than the target number and
the use of the interpolation eliminates the need for accurate pressure
targeting.
(1) Unknown Mode. This general purpose mode is used to measure unknown
samples. Small fixed pressure doses are made at low values of relative
pressure. The pressure of these doses is calculated using the measured
free space of the sample tube and the assumption that the sample has a
minimum surface area. The equilibrated sample pressure and the saturation
pressure are recorded for each dose.
After about six small low pressure data points have been measured, it is
calculated by methods known to those skilled in the art, an additional
amount of gas which must be added in order to achieve the first target
relative pressure point. The calculation provide a one-shot single value
which might or might not provide the desired outcome.
The maximum system pressure and the manifold volume limits the volume of
gas which might be added during each dose. When the calculated amount of
gas that should be added to achieve the target point is more gas than the
system limit, then two or more fully equilibrated data points are taken.
All data points are recorded even if they are not precisely targeted. This
dosing method philosophy can be characterized as try and do better next
time. The more data which has been obtained, the better the isotherm can
be predicted, so that extra data points are not necessary to be obtained.
It is most often the case that ten or less additional data points are taken
when complete adsorption and desorption isotherms are measured. The number
of target relative pressure points required for each analysis is
determined from the required output data. If only a BET surface area is
required, for example, just the first part of the isotherm from 0 to 0.3
relative pressure can be measured.
(2) Learn Mode. This dosing mode obtains a standard isotherm which may be
copied one or more times and is useful for precise targeting of required
targeting of required relative pressure points and for reduced operating
times. The sample measurement throughput is dependent upon many different
factors. The total sample measurement period can be considered to comprise
the following main sections: outgassing of the sample, setup of the sample
measurement conditions, adsorptive dosing time required by the dosing
scheme, sample/adsorptive equilibration time, and sample results and
post-processing time.
The Learn Mode and Copy Mode have not been used before and is particularly
useful for Quality Control (QC) laboratories. A fully detailed analysis is
obtained using a fixed table of relative pressures.
This dosing mode is essentially identical to the Unknown Mode, except that
the target table of relative pressures are fixed and additional data
points are chosen at areas of maximum slope change. The Learn Mode is
slower to perform than the Unknown Mode. The isotherm data obtained from
this mode is stored in the instrument memory. The isotherm data is very
detailed and provides enough information for it to be used as a model for
subsequent analysis of the same type of compound for the Copy Mode.
(3) Copy Mode. This mode can only be used when a Learn Mode run has stored
an isotherm for a similar sample of the same material. The Copy Mode
provides maximum sample throughput for routine quality control analysis.
The target values of the relative pressure are chosen based upon the
required output data and need not be identical to the stored standard
isotherm. Targeted relative pressure points are calculated using the
standard isotherm and the weight of the sample being measured. This mode
eliminates the need for the low pressure fixed dose points. Only the
target table of relative pressure points are employed.
The desorption mode is the reverse of the adsorption mode. The desorption
mode employs a solid sample, referred to as the desorbent, the
morphological characteristics of which are sought to be determined, and a
gas or liquid referred to herein as the desorbate. The desorbate is
desorbed from the sample during the desorption mode, the desorbate being
thereafter referred to as the desorptive. Therefore, the desorption mode
employs a starting sample material, which has been first outgassed as
described herein, and then its surface and any pores present are contacted
with an adsorptive in a manner sufficient to condense the adsorptive on
the sample material, fill the pores, and coat the outer surface of the
sample material with at least a monolayer of condensed desorbate. As a
matter of convenience, the sample material typically is saturated with an
adsorbate to ensure complete filling of the sample pores. Upon the
completion of condensing the gas on the sample material, it is referred to
as the desorbate. Thus, the terms "desorptive" and "desorbate" are used in
place of "adsorptive" and "adsorbate" merely to identify the mode in which
the gas or liquid constituting that substance is employed. The materials
which can constitute the adsorptive and the desorptive are the same. Like
the adsorptive, the desorptive gas can comprise a single component gas and
an inert carrier gas. Preferably, the desorptive gas comprises at least 80
percent of a single component gas and less than 20 percent of a carrier
gas. Most preferably, the desorptive gas comprises a single gas without a
carrier gas.
Accordingly, to conduct the desorption mode, a chamber of known volume and
temperature, as described in accordance with the adsorption mode, is
provided with a sample having desorbate condensed thereon and in
equilibrium with a chamber atmosphere consisting of gaseous desorptive.
This typically is performed by conducting the adsorption mode until sample
saturation is achieved as described hereinabove.
It is to be understood that the present invention is not limited to any
particular mathematical model for using the information embodied in either
the adsorption or desorption isotherm, and such information can be
manipulated as desired in accordance with any conventional procedure.
However, the preferred method to determine the surface area pore volume
and pore size distribution is according to the well known principles as
described in U.S. Pat. No. 4,489,593 to Pieters, et al. For example,
surface area analysis is conducted through determining a number of points
which enable a least squares fit of the BET equation to be accomplished.
The principles, preferred embodiments, and modes of operation of the
present invention have been described in the foregoing specification. The
invention which is intended to be protected herein, however, is not to be
construed as limited to the particular forms disclosed, since these are to
be regarded as illustrative rather than restrictive. Variations and
changes can be made by those skilled in the art without departing from the
spirit of the invention.
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